Dehydrated tomato products comprise an important portion of the tomato industry. The production of tomato pastes, ketchup and other processed tomato products is dependent on the energy-requiring steps of dehydration. The production of “sun-dried” tomato products consists of dehydrating cut tomato fruit either in the sun or in drying ovens.
Dry matter content of mature tomato fruit can range from approximately 5–10% (Davies, J. N. and Hobson, G. E. 1981. The constituents of tomato fruit—the influence of environment, nutrition and genotype. CRC Critical Reviews in Food Sci and Nutr. 15:205–280), depending largely on fruit size. Generally, processing tomato cultivars produce mature fruit with a higher water content of approximately approximate 94–95%. Smaller, “cherry”-type tomato fruit, with a fresh weight of 10–20 grams frequently have higher concentration of solids (dry weight) and hence reduced water concentrations of approximately 90% (10% dry weight).
Generally, tomato fruit development stages can be classified as the pre-climacteric stage, which is comprised of the early stages of fruit growth until incipient ripening, the climacteric stage and the post-climacteric or senescent stage. Once the fruit is fully ripe, tissue disorganization occurs, with pathogens contributing to the tissue disorganization, and characteristics associated with “overripening” and subsequent rotting of the fruit become apparent (Grierson, D. and Kader, A. A. 1986. Fruit ripening and quality. In: Atherton, J. G. and Rudich, J. Eds.: The Tomato Crop. Chapman and Hall, London, pp. 241–280).
The production of raisins from grape berries (Vitis vinifera) is a well known process in which the dehydration process occurs by diffusion of water through a waxy cuticle (Martin, R. J. L. and Scott, G. L. 1957. The physical factors involved in the drying of Sultana groups. Australian Journal of Agricultural Research. 8:444–459). For whole grape berries, the drying process is generally assisted by various dipping treatments of the berry, such as the soda-dip method (Nury, F. S., Brekke, J. E. and Bolin, H. R. 1973. Fruits. In: Van Arsdel, W. B., Copley, M. J. and Morgan, A. I., Eds: Food Dehydration. Avi Publishing Co., Westport, Conn. vol. 2, pp. 158–198). In brief, in this method the berries are dipped in a 0.2–0.3% solution of caustic soda (sodium hydroxide) at a temperature, of about 200° F. for a few seconds and are then rinsed with cold water before dehydration. The purpose of the dipping is to modify the berry cuticle so that transpiration of water vapor across the cuticle may proceed at a faster rate.
The tomato, like the grape, is botanically classified as a berry and has a waxy cuticle on the fruit epidermis (Baker, E. A., Bukovac, M. J. and Hunt, G. M. 1982. Composition of tomato fruit cuticles as related to fruit growth and development. In: Cutler, D. F., and Alvin, K. L. and Price, C. E., Eds: The Plant Cuticle. Academic Press, London, pp. 33–44). However, tomatoes will generally undergo degradation if they remain on the vine after ripening. In the case of tomatoes, the harvested fruit is generally cut in half in order to increase the dehydration rate. Alternatively, whole fruit may be pierced in order to facilitate fluid movement (Ojimelukwe, P. C. 1994. Effects of processing methods on ascorbic acid retention and sensory characteristics of tomato products. J. Food Sci. Technol. 31:247–248). Drying of the slices of pierced tomato fruit may take place either in the sun or in various forms of drying ovens based on non-solar energy input.
There are disadvantages to sun-drying since it depends on weather conditions and inclement weather leads to losses. Similarly, there are disadvantages to the use of drying ovens as these are energy consuming. Both sun drying and oven drying may lead to losses in food quality. For example, levels of ascorbic acid, one of the major nutritional contributions of tomatoes in the human diet, decrease significantly in response to sun-drying or oven-drying (Ojimelukwe, P. C. 1994. Effects of processing methods on ascorbic acid retention and sensory characteristics of tomato products. J. Food Sci. Technol. 31:247–248). Furthermore, the necessity to cut the tomato fruit in half before the drying process does not allow for the production of whole dried tomato fruit.
Wild species of the genus Lycopersicon, such as L. hirsutum, may contain within their genetic makeup expressed characteristics not generally present within the L. esculentum species. These genetic traits may be transferred to the cultivated L. esculentum. For example, the genetic trait of sucrose accumulation is present in mature fruit of the subgenus Eriopersicon (including L. hirsutum, L. chmiliewskii and L. peruvianum) and this trait has been transferred to L. esculentum, using classical genetic breeding techniques, as well as molecular genetic techniques (Schaffer, A. A., Petreikov, M., Miron, D., Fogelman, M., Spiegelman, M., Bnei-Moshe, Z., Shen, S., Granot, D., Hadas, R., Dai, N., Levin, I., Bar, M., Friedman, M., Pilowsky, M., Gilboa, N. and Chen, L. 1999. Modification of carbohydrate content in developing tomato fruit. Hortscience 34:12–14). The wild species of Lycopersicon, however, may also serve as a source of unexpressed genetic traits that can contribute to the value of cultivated plants (Bernacchi, D., Beck-Bunn, T., Eshed, Y., Lopez, J., Petiard, V., Uhlig, J., Zamir, D. and Tanksley, S. 1998. Advanced backcross QTL analysis in tomato. Identification of QTLs for traits of agronomic importance from Lycopersicon hirsutum. Theor. Appl. Genet. 97:381–397).